Conventional mercury fever themometers are being replaced by electronic thermometers which use a temperature-sensitive disposable probe to contact the human body. The probe has a characteristic which varies with temperature, and the characteristic is monitored by suitable circuitry in the electronic thermometer to provide an indication of the sensed temperature on a digital readout such as a liquid crystal display. Such thermometers have a number of advantages including eliminating the hazard of broken glass and uncontained mercury, reducing risk of contamination and eliminating the cost of sterilization. In addition, the temperature reading is quickly displayed in a matter of a few seconds, they are easier to read at low light levels, and they do not need to be shaken down after each use.
Typically, electronic thermometers utilize a probe which is removable and replaceable to provide for decontamination and for replacement of a malfunctioning probe. In some cases the entire probe is disposable. A problem arises when a replacement probe is not properly calibrated for the thermometer. The probes use a heat-sensitive element such as a thermistor and a substantial fraction of the cost of the disposable thermistor probes results from the need to adjust the resistance value of each thermistor element to within an accuracy far greater than can be maintained in production. Two methods are currently used to achieve the required accuracy of resistance of the thermistor elements. The first method consists of adjusting the resistance of each thermistor element by using a laser beam to trim away part of the ceramic wafer until the correct resistance value is achieved. The second method consists of sorting the thermistor wafers after production into matched pairs, each pair having resistance values which are above and below one half of the required value by the same amount, and then using such a matched pair of thermistors in series in each probe instead of a single thermistor. The two thermistors connected in series will have exactly the required resistance value. Both methods involve an extra selection process which is as costly or more costly than the production process itself, and the second method requires in addition the use of two instead of one thermistor element in each disposable probe.